The objective is to demonstrate the feasibility of a membrane-sensor system suitable for use in a minature fiberoptic potassium-ion sensor (optrode) for continuous monitoring in body fluids. Potassium levels are commonly monitored during cardiac surgery, diabetic acidosis, renal failures, and other acute conditions. Potassium levels can be very labile requiring frequent measurements with substantial labor and lab charges. A relatively inexpensive, disposable, and continuous probe would give immediate clinical information and would be cost effective. The specific aims of the Phase I project are: 1. Synthesize a dye molecule that is sensitive to the concentration of potassium-ion in the patho-physiological range. 2. Develop a means of immobilizing the K+ sensor dye molecule within a semipermeable membrane. 3. Construct a working model of the fiberoptic probe. 4. Determine effectiveness of probe by in-vitro analysis of body fluid with known K+ concentrations. The longer term Phase II objectives are to minaturize the K+ probe and combine it into a multiparameter probe for monitoring of several components such as 02, C02, and pH.